A two-stroke engine, as compared to a four stroke engine, has the advantages of simple structure, low manufacturing cost, easy maintenance and steady output of horse power. However, it also has the disadvantages of higher fuel consumption and higher air pollution. Since they have such an impact on air pollution, the public has occasionally recommended a prohibition of two-stroke engines.
In the conventional two-stroke engine, a fuel-air mixture is charged into the cylinder. After the compression stroke of the piston the mixture is ignited. The exhaust gases are forced out of the cylinder through an exhaust port by the exhaust stroke of the piston while the fresh fuel-air mixture is added. The exhaust port remains partially open while the piston is completing the exhaust stroke, so some of the fresh fuel-air mixture escapes into the exhaust port. When the engine is operating at large throttle opening, more fuel-air mixture is lost through the exhaust port because of the strong suction in the expansion chamber which is created by expansion of waste gas and its kinetic energy.
The amount of fuel-air mixture lost during operation of a conventional two-stroke engine is within the range of 25% to 40%. This loss of fuel reduces the fuel economy of the engine. The escaped fuel-air mixture also worsens the pollution caused by the engine. The gas which escapes from the cylinder during the final stage of the exhaust stroke contains more hydrocarbons, and has a higher density of burnable gas, than the prior exhaust gas. An engine which reduces the amount of unburned fuel which escapes from the cylinder would have a better fuel economy, and less emission.